Pre-passivation-anodic inhibitor-color method for detecting cracks in metal bodies

ABSTRACT

A non-destructive color method for detecting cracks in a metal body by selective corrosion of the surface portion within the cracks which includes pre-passivation of the surface being tested to substantially reduce background indications. Specifically, a passivating electrolyte solution containing an anodic inhibitor is applied to the surface of the metal body to be tested and removed therefrom after a period of time sufficient to passivate its open surface portion but insufficient to significantly passivate the surface portion contained within cracks therein. A color-forming electrolyte indicating solution is then applied to the thus-passivated surface area. The color-forming electrolyte indicating solution contains halide ions, a color-forming indicator and an anodic inhibitor and is formulated so that it does not corrode the open surface portion of the metal body but is sufficiently reactive to corrode the surface portion within cracks therein resulting in the formation of metallic ions. The color-forming indicator is of the type which reacts with the thus formed metal ions to form a distinctive colored compound at the crack locations.

United States Patent Henry Appl. No.: 889,693

US. Cl. ..l48/6.2, 23/230 C, 23 230 L/, 148/62, 252/408 Int. Cl ..C23f 9/00, G0ln 31/22 Field ofSearch ..148/6.14, 6.15, 6.16, 6.2; 1 17/63, 66, 47 R, 8; 252/408; 73/150; 23/230 C,

References Cited UNITED STATES PATENTS 2/1944 De Forest ..252/408 X 4/l955 De Forest ..252/408 X [451 May 23, 1972 Primary ExaminerRalph S. Kendall Assistant Examiner-Caleb Weston ArtorneyCharles T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT A non-destructive color method for detecting cracks in a metal body by selective corrosion of the surface portion within the cracks which includes pre-passivation of the surface being tested to substantially reduce background indications. Specifically, a passivating electrolyte solution containing an anodic inhibitor is applied to the surface of the metal body to be tested and removed therefrom after a period of time sufiicient to passivate its open surface portion but insufficient to significantly passivate the surface portion contained within cracks therein. A color-fonning electrolyte indicating solution is then applied to the thus-passivated surface area. The color-forming electrolyte indicating solution contains halide ions, a colorforming indicator and an anodic inhibitor and is formulated so that it does not corrode the open surface portion of the metal body but is sufficiently reactive to corrode the surface portion within cracks therein resulting in the formation of metallic ions. The color-forming indicator is of the type which reacts with the thus formed metal ions to form a distinctive colored compound at the crack locations.

12 Claims, No Drawings PRE-PASSIVATION-ANODIC INHIBITOR-COLOR METHOD FOR DETECTING CRACKS IN METAL BODIES This invention relates to a non-destructive method for detecting cracks in a metal body. In particular, the invention contemplates a process resulting in improved economy and versatility for inspection of metal bodies whereby a clear, precise, reproducible record of the inspection results is rapidly obtained.

Metal fatigue causes 75 percent of all the serious engine failures in jet aircraft today. If adequate inspection techniques were available, these failures could be prevented. The fatigue process is a slow one; early in the process small microcracks are nucleated either at grain boundaries intersecting the surface, at hard precipitate particles at or near the surface, or other surface flaws. These microcracks grow at a very slow but accelerating rate leading to a large crack which will eventually cause catastrophic failure if it remains undetected. Since the fatigue crack grows slowly, there is usually ample opportunity to detect the crack during inspection periods before it grows to a critical size.

Current crack detection techniques rarely are able to detect cracks smaller than three mils in depth, and in most circumstances to mils is usually the lower limit of detectability. With the more traditional structural alloys and conservative design, a crack can grow to much larger than this detection threshold before it becomes critical in size. But in the newer, high temperature alloys, which have a much lower toughness and are designed for much higher stress levels, the critical crack size begins to approach the limit of detectability. Under these conditions detection techniques of the prior art are not adequate.

A typical detection process widely used in industry utilizes penetrants known in the art which include various liquids mixed with a dye in combination with a developer or blotting agent. The use of such a liquid penetrant generally comprises application of the penetrant to the surface of the metallic specimen followed by a soaking period of at least about 30 minutes to permit penetration into cracks, if any. The excess penetrant is then removed usually with a suitable solvent. A developer or blotting agent which contains a powder such as ground silica or talc, is then applied to the surface to provide a contrasting background to make the dye more visible and to blot the liquid dye out of the surface cracks by absorption. The pattern of blots shows the general location of cracks as well as other surface discontinuities.

These dye penetrant inspection procedures of the prior art have several drawbacks. They require a number of steps and are time consuming. The effectiveness of the liquid penetrant depends on its wetting properties. The penetrant must not only be able to flow into a crack but also to be absorbed out of it by a developer, and therefore, cracks almost closed at their mouths would not be detected in most instances. In addition, known liquid penetrants are limited regarding temperatures at which they may be used and begin losing their penetrating power at temperatures below 70 F due to varying surface tension and flow characteristics. Also, the blotting action of the developer depends upon its absorption of the liquid dye penetrant. Since this absorbing action continues as long as liquid dye remains in contact with the developer, the pattern initially produced quickly becomes fuzzy and definition is lost in a matter of minutes. Moreover, the definition obtained in the dye pattern depends partly upon grain size of the developer, and since larger grain sizes produce poorer definition, developers of very uniform small grain size are required. In addition, the test specimens must be handled with care to prevent wiping off of the developer. Also, and most importantly, the mass of blots produced by conventional penetrants may be the result of normal voids or surface scratches, making the detection of structural microcracks difficult.

To avoid the use of developers, some prior art methods use a fluorescent penetrant. Typically, this comprises applying to the surface of the test body the luminescent penetrant, removing the penetrant remaining on the surface generally by solvent cleaning, and then inspecting the surface of the body in darkness under fluorescigenous radiation such as ultraviolet light or black light for the portion of the penetrant which had penetrated into the surface openings. The particular problem with this type of method is that many large castings and like bulky articles are not readily transported to locations which may be darkened as required for inspection. Such heavy bodies are also handled with considerable difficulty during the inspection procedure once the location is satisfactorily darkened. In addition, the sources of the required fluorescigenous radiations, almost exclusively high intensity mercury vapor arcs, are relatively expensive special equipment. Also, some operators required to inspect test bodies continuously under such sources complain of the undesirable effect of continued eyeball fluorescence. In some applications the method is too sensitive in that it is difficult to distinguish between normal surface irregularities and structural cracks, and the result has been over-inspection and excessive rejection by inexperienced operators.

The drawbacks of the prior art methods are overcome by the process disclosed and claimed in copending US. Pat. application Ser. No. 889,699 entitled Anodic Inhibitor-Color Method For Detecting Cracks In Metal Bodies filed of even date herewith in the names of Louis F. Coffin, Jr. and Lyman A. Johnson and assigned to the assignee hereof, and which, by reference, is made part of the disclosure of the present application. The process of this copending application comprises applying to the surface of a metal body to be tested a colorforming electrolyte which forms a distinctive colored compound at crack locations in the body. Specifically, the colorforming electrolyte is an aqueous solution containing halide ions, a color-forming indicator and an anodic inhibitor. It is formulated so as not to corrode the open surface portion of the metallic material, but to corrode the internal surface portion contained within the cracks resulting in the formation of metallic ions within the cracks. The color-forming indicator is of the type which reacts with the thus formed metal ions to produce a distinguishing colored compound. This process can be carried out in normal light in a matter of minutes, generally about fifteen minutes, and frequently, within shorter periods of time and does not use a developer or blotting agent. In addition, it is particularly satisfactory for detecting cracks which are tightly closed at their mouths. This is of particular importance since parts are inspected usually in an unstressed, cold condition which commonly leaves the cracks tightly closed.

According to the present invention, a particularly clear indication of cracks can be obtained with the color process disclosed in the copending application by initially treating the surface of the metal to be tested to passivate it. Specifically, roughness or normal voids in the surface frequently cause spotty background of varying color intensity in the test for cracks and make detection of the cracks, themselves, more difiicult. The present process substantially reduces such a background by initially passivating the surface to be tested, preferably using a passivating solution which is the same as the color-forming electrolyte solution used to detect cracks.

Briefly stated, the process of the present invention comprises a passivating step in combination with the color process disclosed in the copending application. Specifically, a thin coating of a passivating solution is applied to the surface to be tested, and after a short period of time, which is long enough to passivate the open surface portion of the body but not sufficient to allow significant passivation of the surface portion contained within cracks, the coating is removed. The colorforming electrolyte is then applied to the thus-passivated surface area, and within a short period of time, forms a distinctive colored compound at the crack locations in the body.

In carrying out the process of the present invention, the surface of the metal body to be tested should be initially cleaned in a conventional manner to reduce background indications so that a clearer and sharper indication of cracks can be produced. Conventional methods of cleaning metals can be used such as by wiping the surface with a suitable organic solvent to remove oils and grease or by mild grit blasting to remove adherent surface contaminants, such as paint and scale.

The surface to be tested is then coated with the present passivating solution which after a specific short period of time is removed. The particular composition of the passivating electrolyte solution depends largely on the type of metal being tested. It is always an aqueous solution containing an anodic inhibitor. The passivating solution should not be of a composition, i.e. corrosive strength, which would cause any significant corrosion of the surface area being passivated, i.e. the open or free surface portion of the metal body. It should, however, be sufficiently reactive or strong to passivate the open surface portion of the metal body so that in the subsequent testing for cracks background indications caused by roughness or normal voids are substantially reduced. The anodic inhibitor concentration of the passivating solution generally falls within the same range as that useful in forming the color-forming electrolyte solution used for detecting cracks. Preferably, the passivating electrolyte solution has the same composition as the color-forming electrolyte solution used subsequently to detect cracks.

It is believed that the present passivating electrolyte solution passivates the background causing areas of the surface by forming a passive film thereon. The passivating solution is coated on the surface to be tested to form a thin continuous film. It can be applied in a conventional manner. Preferably, a coating is applied by saturating an absorbent material such as cotton with the passivating solution and rubbing it onto the surface. After a short eriod of time which can be determined empirically, and which generally ranges up to about -30 seconds, the coating is removed with a conventional material such as cloth or paper so as not to break to any significant extent the just-formed passive film. The passivating electrolyte coating should be wiped off before any significant amount ofit enters a crack and thereby significantly passivates the surface portion therein. Significant passivation of the surface portion contained within the cracks would inhibit or slow down the subsequent detection of the cracks by the color-forming electrolyte solution. On the other hand, in some instances, the coating of the passivating solution can be allowed to air dry if such coating does not passivate the surface portion within the cracks to the extent that it later interferes with crack detectron.

After such passivation, cracks are detected in the metal body by selective corrosion of the surface portion within the cracks. By corrosion, it is meant herein the dissolution of the metal by its liquid environment. The bodies of metallic material which are useful in the present invention are those which, because of the presence of the anodic inhibitor in the colorforming electrolyte indicating solution, will exhibit what is generally known in the art as the phenomenon of crevice corrosion. Representative of these metallic materials are iron, low alloy steels, medium alloy steels and iron-carbon steels.

The color-forming electrolyte indicating solution is always an aqueous solution containing halide ions, a color-forming indicator and an anodic inhibitor, and its specific formulation is determinable empirically. The anodic inhibitor is used in an amount within a range sufficient to passivate the open surface portion of the metallic material so that it will exhibit the phenomenon of crevice corrosion, i.e. it should not be used in an amount which would passivate to a significant extent the surface portion within a crack in the material. Typical anodic inhibitors useful in the color-forming electrolyte solution as well as the passivating solution include sodium dichromate,

sodium chromate, potassium chromate, potassium dichromate, sodium bicarbonate, borax, pyrophosphate and calcium carbonate. The specific anodic inhibitor used depends largely on the metallic material to be tested. In general the amount of inhibitor should be very small, of the order of a few tens of parts per million, and the lower the halide ion concentration, the lower should be the inhibitor concentration. Specifically,

the concentration of the anodic inhibitor in the color-forming electrolyte solution as well as the passivating electrolyte solution generally may range from about 0.0001 Molar or lower to about 0.05 Molar.

The halide ions useful in the present process can be chloride, bromide, iodide and fluoride ions. Chloride ions produce the best results and are preferred. Usually, for a majority of metallic materials of the present invention, the halide ions in the passivating electrolyte solution as well as the color-forming electrolyte solution are provided by a metal halide salt. Representative of these salts are sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium iodide and potassium iodide. The salt should be used in the color-forming electrolyte aqueous solution in an amount sufficient to provide enough halide ions to accelerate corrosion within cracks to result in a colored compound being formed in a reasonable period of time, generally within about 15 minutes. However, the salt should not be used in an amount which would cause the color-forming electrolyte to corrode the open surface portion of the metal to any significant extent. In addition, the metal halide salt used in the passivating solution as well as the color-forming solution should be selected so that its metal ions do not form a colored compound with the color-forming indicator. The concentration of the metal salt used to provide the halide ions in the passivating electrolyte solution as well as the color-forming electrolyte solution can vary widely depending largely upon the particular salt used and the metal being tested but it generally ranges from about 0.05 to about 2 Molar.

Upon application of the color-forming electrolyte indicating solution to the surface to be tested, it acts to corrode the surface portion within cracks to cause the formation of metal ions. The resulting positively charged metal ions hydrolyze in the surrounding electrolyte in the crack, increasing its acidity and thereby increasing the corrosion rate within the crack. In addition, the positively charged metal ions formed within the crack attract some of the halide ions of the electrolyte which accelerates corrosion therein. The color-forming indicator reacts with the metal ions produced by corrosion in the crack to form a distinctive colored compound at the crack locations in a matter of minutes, generally up to about fifteen minutes and frequently within a shorter period of time.

The corrosive strength of the color-forming electrolyte is determined largely by the difference in the rate of corrosion between the open surface portion of the metallic material and the surface portion within a crack therein, i.e. it should be sufficiently strong to corrode the surface portion within a crack but not the open surface portion of the material to any significant extent. The composition of the color-forming electrolyte is not significantly changed at the free or open surface portion but its composition does change at the surface portion within a crack becoming increasingly acidic. This is due to a number of factors such as, for example, a lack of oxygen within the crack. The useful halide ion content of the color-forming electrolyte generally falls within a specific range for a particular metallic material so that the distinctive colored compound which indicates the crack locations is formed in a reasonable period of time, generally within about 15 minutes. Stronger color-forming electroltyes are preferably used where cracks of very small depth must be detected since these cracks corrode at a rate slower than deeper cracks. In addition, the stronger electrolyte, i.e. more corrosive, within a range are preferably used to locate cracks which are closed at their mouths. In this instance, the color-forming electrolyte solution enlarges the mouth of the crack by corrosion and flows in.

For some metallic materials of the present invention, the halide ions can be provided by hydrochloric, hydrobromic, hydroiodic or hydrofluoric acid provided the acid is sufficiently dilute in each electrolyte so that it does not corrode the open surface portion of the metal. In addition, in this instance, the color-forming electrolyte should corrode the surface portion within a crack therein to form a colored compound in a reasonable period of time, generally within about 15 minutes.

The color-forming indicator is one which reacts with the metal ions formed by corrosion to produce a distinctive colored compound at crack locations. For example, in the case of iron, low and mild alloy steels, ferrous ions are formed by corrosion within the crack. One of the most satisfactory color-forming indicators for ferrous ions is potassium ferricyanide K;,Fe(CN) which reacts with the ferrous ions to form a deep blue precipitate of ferrous ferricyanide known as Tumbulls blue. The reaction is as follows:

Additionally, ferric ion reacts with any ferrocyanide ion present to form a precipitate of Prussian blue as follows:

Color-forming indicators for use with specific metal ions are well known in the literature. For example, an extensive list is disclosed in Feigl, F Qualitative Analysis By Spot Tests, 3rd ed., Elsevier Pub., NY. 1946.

The amount of color-forming indicator is determinable empirically, and generally, amounts resulting in a concentration in the color-forming electrolyte solution, as well as the passivating solution, of about 0.0005 to about 0.01 Molar are satisfactory. 1

In carrying out the process, the color-forming electrolyte may be applied on the surface area of the metal body to be tested in a conventional manner such as from an eye dropper or other conventional means. After a short period of time, a distinctive colored compound will become visible at the crack locations. If desired, the part to be tested may be immersed in the color-forming electrolyte, and after a short period of time, a colored compound will become visible at crack locations.

The colored compound which indicates crack locations is easily removed. It may be removed by simply wiping it offwith a cloth or paper such as Kimwipe. Generally, it can be removed most readily by applying to the colored-compound carrying surface a complexing agent and then wiping off the complexed compound. Typical complexing agents include ethylene diamine tetraacetic acid and Biguanide [H NC(:NH) NH.

A wetting agent can be included in the passivating electrolyte solution as well as the color-forming electrolyte solution to increase their penetrating or wetting powers. Conventional wetting agents can be used and they may be anionic, cationic or nonionic. The wetting agent should not be decomposed or degraded by the electrolytes to any significant extent. The amount of wetting agent used is not critical and may vary within a wide range depending on the increased penetration or wetting produced by a specific amount. Generally, satisfactory results are obtained with amounts of wetting agent ranging from about 0.0l to about 2 percent by volume of the passivating electrolyte solution as well as the color-forming electrolyte solution. Representative of the anionic wetting agents are sodium salts of organic sulfonates, especially alkylaryl sulfonates such as the sulfonates of dodecylbenzene, as for example, disodium-4-dodecylated oxydibenzenesulfonate. Other representative anionic surfactants include sodium alkylnapththalenesulfonate, sodium N-methyl-N-oleyltaurate, sodium oleylisethionate and the sodium salt of sulfated nonyl phenoxypoly (ethyleneoxy) ethanol. Typical cationic surfactants include lauryltrimethylammonium chloride and octadecyltrimethylammonium chloride. Examples of nonionic surfactants are polyethylene glycol lauryl ether and tris polyoxyethylene) sorbitan monolaurate.

The invention is further illustrated by the following examples.

In the following examples, an AISI-SAE 4140 steel, a low alloy steel, was used. The allow is composed of C0.38 0.43%, Mn-0.75 1.00, P-0.040 max., 5-0.040 max., Si0.20 0.35, Cr-0.80 L10, Mo-0. l5 0.25 and Fe balance. This metallic material is used in the aircraft industry as a compressor disc alloy which is an application that undergoes considerable fatigue.

EXAMPLE 1 A bar of the steel, 8 inches long, 0.5 inch wide and 0.125 inch thick was cut and a gage section .was carefully surface ground in one direction. The bar was then fatigued by reverse bending to cause microcracks in the direction normal to that of the grinding direction. These microcracks could be seen under a high powered microscope. (In normal operation, the cracks would not be distinguishable under a microscope since they usually occur in the same direction as the surface grinding marks).

An aqueous electrolyte was formed which was comprised of l Molar sodium chloride, 0.005 Molar sodium dichromate (Na QQQZ-ZHZQ).andOQSEUM Iar.Pq ferricyanide This electrolyte was used as the passivating electrolyte as well as the color-forming indicating electrolyte.

The electrolyte solution was applied to one surface of the gage section of the bar at room temperature from an eye dropper to cover the entire surface. Within about one minute a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. However, there was also considerable background indication of varying blue colored intensity.

For comparison purposes the above procedure was repeated on the opposite side of the gage section of the bar, having the same appearance as that tested above except that this surface was initially passivated with the electrolyte solution. Specifically, a clean cotton cloth was saturated with the electrolyte solution and was then rubbed across the surface area for about IO seconds to form a thin continuous film thereon. After an additional period of about 15 seconds, it was wiped off with Kimwipe paper. The electrolyte solution was then applied to the thus treated surface area of the gage section of the bar from an eye dropper to cover the entire treated surface. Within about one minute a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. The background indications were substantially reduced. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surfaces tested.

EXAMPLE 2 The procedure used in this example was the same as that set forth in Example 1 except that another bar was used and the electrolyte solution was comprised of 0.1 Molar sodium chloride, 0.002 Molar potassium ferricyanide, 0.005 Molar sodium dichromate (Na Cr O 2H O) and 0.125 Molar hydrochloric acid. Within about one minute, a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. A significant reduction in background indications resulted by initially passivating the surface with the solution. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surface tested.

EXAMPLE 3 The procedure used in this example was the same as that set forth in Example 1 except that another bar was used and the electrolyte contained about 0.1 ml of a wetting agent sold under the trademark Aquet which is a nonionic alkyl aryl polyethylene glycol liquid. Within about 1 minute, a deep blue precipitate occurred at the crack locations and was clearly visible in daylight. Initial passivation of the surface resulted in a substantially reduced background. The blue precipitate was then cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surface tested.

In copending US. Pat. application Ser. No. 889,694 entitled Pre-Passivation-Color Method For Detecting Cracks In Metal Bodies" filed of even date herewith in the name of Michael F. Henry and assigned to the assignee hereof there is disclosed a process combining pre-passivation with a color method for detecting cracks in metal bodies wherein a colorforming aqueous acid indicating solution containing halide ions and a color-forming indicator is used.

In copending US. Pat. application Ser. No. 889,696 entitled Color Method For Detecting Cracks ln Metal Bodies filed of even date herewith in the names of Louis F. Coffin. Jr and Lyman A. Johnson and assigned to the assignee hereof there is disclosed a color method for detecting cracks in a metal body using a color-forming aqueous indicating solution which contains halide ions and a color-forming indicator.

ln copending US. Pat. application Ser. No. 889,695 entitled Method For Detecting Cracks In Metal Bodies" filed of even date herewith in the names of Michael F. Henry and Lyman A. Johnson and assigned to the assignee hereof there is disclosed the selective corrosion of the surface portion within cracks to enlarge them sufficiently so that they can be detected by a conventional technique using a conventional liquid.

All of the above cited patent applications are, by reference, made part of the disclosure of the present application.

What is claimed is:

1. A color process for detecting cracks in a metal body by selective corrosion of the surface portion within the cracks which comprises applying to the surface of the metal body to be tested a passivating electrolyte aqueous solution containing halide ions and containing an inorganic anodic inhibitor, said passivating solution being allowed to remain on said surface for a period of time sufficient to passivate the open surface portion of the metal body but insufficient to significantly passivate the surface portion contained within cracks therein, applying to the thus-passivated surface of the metal body a colorforming electrolyte aqueous indicating solution containing halide ions, a color-forming indicator and an inorganic anodic inhibitor. said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to produce metal ions, and said color-forming indicator being reactive with the resulting metal ions to produce a colored compound.

2. A process according to claim 1 wherein said passivating electrolyte solution contains a color-forming indicator.

3. A process according to claim 2 wherein the composition of said passivating electrolyte solution and said color-forming electrolyte indicating solution is comprised of a metal halide salt in a concentration of about 0.05 to 2 Molar, a color-forming indicator in a concentration of about 00005 to about 0.01 Molar, and an anodic inhibitor in a concentration of about 0.0001 to about 0.05 Molar.

4. A process according to claim 3 wherein said passivating electrolyte solution has the same composition as said colorforming electrolyte indicating solution.

5. A process according to claim 1 wherein the metal body is selected from the group consisting of iron, low alloy steels, medium alloy steels and plain carbon steels.

6. A process according to claim 1 wherein said halide ions are provided by a metal salt.

7. A process according to claim 6 wherein said metal salt is sodium chloride.

8. A process according to claim 1 wherein said passivating electrolyte solution contains a wetting agent.

9. A process according to claim 1 wherein said color-forming electrolyte indicating solution contains a wetting agent.

10. A process for detecting cracks in an iron alloy metal body by selective corrosion of the surface portion within the cracks which comprises applying to the surface of the metal body a passivating electrolyte aqueous solution of sodium chloride, potassium ferricyanide and sodium dichromate, said passivating solution being allowed to remain on said surface for a period of time sufficient to passivate the open surface portion of the metal body so that background indications are substantially reduced when testing for cracks but insufficient to significantly passivate the surface portion contained within cracks therein, applying to the thus-passivated surface of the metal body a color-forming electrolyte aqueous indicating solution of sodium chloride, potassium ferricyanide and sodium dichromate which is formulated so that it does not corrode the open surface portion of the metal body but 18 reactive with the surface portion within the cracks to produce ferrous ions and said potassium ferricyanide reacting with the resulting ferrous ions to produce a blue precipitate, said passivating solution and said color-forming indicating solution being comprised ofsodium chloride in a concentration of about 0.05 to 2 Molar, potassium ferricyanide in a concentration of about 0.0005 to about 0.01 Molar and sodium dichromate in a concentration of about 0.0001 to about 0.05 Molar.

11. A process according to claim 10 wherein said passivating electrolyte solution has the same composition as the said color-forming electrolyte indicating solution.

12. A process according to claim 11 wherein each said electrolyte solution contains a wetting agent. 

2. A process according to claim 1 wherein said passivating electrolyte solution contains a color-forming indicator.
 3. A process according to claim 2 wherein the composition of said passivating electrolyte solution and said color-forming electrolyte indicating solution is comprised of a metal halide salt in a concentration of about 0.05 to 2 Molar, a color-forming indicator in a concentration of about 0.0005 to about 0.01 Molar, and an anodic inhibitor in a concentration of about 0.0001 to about 0.05 Molar.
 4. A process according to claim 3 wherein said passivating electrolyte solution has the same composition as said color-forming electrolyte indicating solution.
 5. A process according to claim 1 wherein the metal body is selected from the group consisting of iron, low alloy steels, medium alloy steels and plain carbon steels.
 6. A process according to claim 1 wherein said halide ions are provided by a metal salt.
 7. A process according to claim 6 wherein said metal salt is sodium chloride.
 8. A process according to claim 1 wherein said passivating electrolyte solution contains a wetting agent.
 9. A process according to claim 1 wherein said color-forming electrolyte indicating solution contains a wetting agent.
 10. A process for detecting cracks in an iron alloy metal body by selective corrosion of the surface portion within the cracks which comprises applying to the surface of the metal body a passivating electrolyte aqueous solution of sodium chloride, potassium ferricyanide and sodium dichromate, said passivating solution being allowed to remain on said surface for a period of time sufficient to passivate the open surface portion of the metal body so that background indications are substantially reduced when testing for cracks but insufficient to significantly passivate the surface portion contained within cracks therein, applying to the thus-passivated surface of the metal body a color-forming electrolyte aqueous indicating solution of sodium chloride, potassium ferricyanide and sodium dichromate which is formulated so that it does not corrode the opeN surface portion of the metal body but is reactive with the surface portion within the cracks to produce ferrous ions and said potassium ferricyanide reacting with the resulting ferrous ions to produce a blue precipitate, said passivating solution and said color-forming indicating solution being comprised of sodium chloride in a concentration of about 0.05 to 2 Molar, potassium ferricyanide in a concentration of about 0.0005 to about 0.01 Molar and sodium dichromate in a concentration of about 0.0001 to about 0.05 Molar.
 11. A process according to claim 10 wherein said passivating electrolyte solution has the same composition as the said color-forming electrolyte indicating solution.
 12. A process according to claim 11 wherein each said electrolyte solution contains a wetting agent. 